Piccolo Singleton Manager

singleton.rs

pub struct LuaSingletons {
    singletons: Rc<AtomicCell<HashMap<usize, Box<dyn Any>>>>,
}
impl Default for LuaSingletons {
    fn default() -> Self {
        Self {
            singletons: Rc::new(AtomicCell::new(HashMap::default())),
        }
    }
}
impl LuaSingletons {
    /// Fetch a lua singleton, initializing it if it has not yet been created.
    ///
    /// The singleton is defined by a function pointer that returns a stashable value.
    fn get<
        'gc,
        S: Fetchable<'gc, Fetched = T> + 'static,
        T: Stashable<'gc, Stashed = S> + Clone + Copy + 'gc,
    >(
        &self,
        ctx: Context<'gc>,
        singleton: fn(Context<'gc>) -> T,
    ) -> T {
        let map = self.singletons.borrow_mut();
        let id = singleton as usize;
        if let Some(entry) = map.get(&id) {
            let stashed = entry.downcast_ref::<S>().expect(
                "Encountered two functions with different return types and \
                the same function pointer.",
            );
            ctx.state.registry.fetch(stashed)
        } else {
            drop(map); // Make sure we don't deadlock
            let v = singleton(ctx);
            let stashed = ctx.state.registry.stash(&ctx, v);
            self.singletons.borrow_mut().insert(id, Box::new(stashed));
            v
        }
    }
}